Basic facts:

2,7 grammes of sugar gives 1 hydrometer degree in 1 litre of liquid.

17 grammes of sugar gives 1% of alcohol in one litre of wine.

Wine yeast only tolerates 17-18% alcohol, after which it rests.

A reading of over 1090 Hydrometer degrees at the start or 1075 degrees during fermentation can stop the process.

Proceed as follows:

Start the fermentation at approx. 1080 degrees (4 kg sugar in 20 litres).

When 1040 degrees has been fermented to, add the remaining sugar (about 2 kg).